The present invention relates to a synthetic silica glass article used in the process of dopant diffusion in semiconductors. More particularly, the invention relates to an article used in the process of dopant diffusion in semiconductors made from synthetic silica glass having a very high purity and exhibiting outstandingly low creeping deformation even at high temperatures encountered in the process.
Needless to say, it is essential that various articles and tools used in the process of dopant-diffusion in semiconductor processing, such as diffusion process tubes, wafer baskets, wafer boats, cantilevers and the like, referred to as a doping article hereinbelow, have a purity as high as possible and are free from the troubles of deformation by creeping even by prolonged use at a high temperature. Fused silica glass is the most conventional material for such articles although silica glass is subject to creeping deformation or warping at a temperature exeeding, for example, 1200.degree. C. Therefore, it is usual that an article or tool to be used at 1200.degree. C. or higher is made from silicon carbide which is much more heat-resistant than fused silica glass so that the article made therefrom is almost free from thermal deformation or warping.
Silicon carbide, however, is not always quite satisfactory as a material for the fabrication of doping articles in respect of the poor fabricability into complicated forms and relatively high impurity content. Silica glass is traditionally obtained from a pulverized and refined powder of natural quartz crystals by fusion into a melt from which pipes, rods and the like are prepared by drawing. A problem in such natural fused quartz glass is again the relatively high content of impurities even after an elaborate refining process, for example, utilizing an electrolytic method.
In view of the above mentioned problem in the fused silica glass prepared from natural quartz, it is a trend in recent years that silica glass is prepared more and more from synthetically obtained silicon dioxide. Though advantageous in respect of the high purity and availability in any large quantities, synthetic silica glass is defective in the relatively low heat resistance which limits the highest temperature, at which a doping article made therefrom can be used without the troubles of thermal creeping or deformation, to about 1000.degree. C. Several proposals have been made to decrease thermal creeping of synthetic silica glass by admixing the melt of silica with chromium oxide, molybdenum oxide, zirconium oxide, iron oxide and the like (see U.S. Pat. No. 4,028,124) or with a combination of aluminum oxide and elementary silicon (see U.S. Pat. No. 4,047,966) to increase the rigidity of fused silica glass at high temperatures. Such a method is not always applicable to the silica glass for doping articles because such additive elements may act as a contaminant of the semiconductor materials under processing by using the doping article.